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10 SMT007 MAGAZINE I MAY 2019 In this installment of my column series on the role of bismuth (Bi) in electronic products, I'll look at the effects of Bi on the properties and performance of solder interconnections in electronic products when Bi is not contained in the solder alloy for the SMT assembly pro- cess (Bi-absent solder alloy composition of sol- der paste). The effects of Bi in solder joints are created by an unintentional path through the supply chain, which introduces Bi into the resulting solder alloy of solder joints. The per- formance and reliability of the resulting solder joint impacted by the presence of Bi can vary; it can be beneficial or detrimental or no-detect- able-effect. From the supply chain in electronics assem- bly, Bi can come from component lead coat- ing, passive component termination coating, and PCB surface finishing that are Bi-con- taining material (albeit, this has not been a common PCB surface finish in recent years). Accordingly, even in a Bi-absent assembly pro- cess (e.g., using SAC solder paste or SnPb sol- der paste), the changes in the properties and performance of solder joints due to the intro- duction of Bi into the solder joint could occur. Similarly, for BGA components, the Bi-contain- ing solder ball will make Bi-containing solder joint. In SMT assembly after reflow, the composi- tion of solder joints is expected to deviate from the composition of the solder alloy used in sol- der paste. The compositional change in solder joints as a result of Bi contributions from one or both of component and PCB surface finish should not be dismissed. Bi contribution from component leads (component surface coating) to the composition of the solder joint, while The Role of Bismuth (Bi) in Electronics, Part 6 SMT Prospects & Perspectives by Dr. Jennie S. Hwang, CEO, H-TECHNOLOGIES GROUP

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